This invention relates to a method for making thin film superconducting circuits of niobium and niobium-containing compounds. More specifically this invention relates to an improved method for etching thin films of niobium and niobium-containing compounds for preparing superconductive circuits. The invention also relates to an improved method for lifting off the circuits from the substrate.
As computers become more complex, there is a trend toward the development of superconducting circuits for use in the computers of the future. For example computer circuits, which are switches, generate heat as they open and close. The faster the switches operate, the more heat they develop. Some of the fastest switches produce so much heat that they must be placed relatively far apart in order that they will continue to work.
To overcome the heat, high speed superconducting circuits using components such as the Josephson junction have been developed. These circuits may be prepared by applying a thin film of about 0.25 to 2.0 microns of superconductive material such as Nb.sub.3 Sn or Nb.sub.3 Ge to an appropriate smooth substrate(s) such as Pyrex, fused silica or Al.sub.2 O.sub.3 (sapphire). The films may be applied to the substrate by any suitable means for depositing thin films such as sputter deposition, chemical vapor deposition, magnetron sputtering, or electron beam gum evaporation on a heated substrate. A layer of photoresist material is deposited on top of the film. A pattern of the desired circuit is then placed on the photoresist material which is then exposed and developed, resulting in a mask of the circuit pattern on the surface of the film. The circuit is then produced by contacting the film with an etchant solution which is supposed to dissolve only the unmasked film, leaving only the desired circuit as a thin film on the substrate surface.
However, niobium and superconducting niobium compounds are very inert so that it is a problem to etch the film at a rate which is fast enough to completely remove all of the excess film from the substrate without destroying the photoresist mask and consequently the circuit. It is also important not to etch the film at a rate which is too fast and hence difficult to control since it might undercut the photoresist mask, etch away too much of the film and destroy the circuit. Thus it is a difficult problem to find an etchant which can be controlled so that it will only remove the unmasked niobium and niobium compound film.